It is well known to use an OTDR to determine or monitor loss characteristics of an optical fiber in an optical communications system. With increasing use of optical communications over both long and short distances, and with extension of optical fiber communications to subscribers' premises and local area networks, there is an increasing need for such monitoring in a convenient and effective manner.
Optical fibers which are currently used in optical communications systems are either single mode or multi-mode fibers. With single mode fibers, one mode (LP.sub.01) is propagated over large distances with relatively little attenuation or loss, whereas other, higher order, modes are heavily attenuated over such distances. Accordingly, single mode fibers are used for relatively long optical fiber communications paths, for which low attenuation and low dispersion of the optical signal are required. Multi-mode fibers, in which a number of different modes are simultaneously propagated along the fiber, tend to be used for relatively short optical fiber communications paths, for example in optical fiber local area networks (LANs), where they provide advantages such as lower costs and easier splicing due to their larger core size.
As is well known, in an OTDR light from an optical source such as a laser diode is coupled to a fiber to be tested, and light reflected back via the fiber (back-scattered light) is detected to provide a display of fiber attenuation characteristics along the length of the fiber. By way of example, Maeda U.S. Pat. No. 4,737,027 issued Apr. 12, 1988 and entitled "Optical Fiber Measuring Apparatus" illustrates such an OTDR arrangement using lenses for coupling the light between laser diodes, detector, and a connector for the fiber. In practice, such a lens system is difficult to implement, and accordingly it is normal to use optical fibers for communicating the optical signals among the components of the OTDR. For example, Dalgoutte et al. U.S. Pat. No. 4,737,026 issued Apr. 12, 1988 and entitled "Plural Wavelength Optical Fibre Reflectometer" describes an OTDR for single mode fiber in which single mode fibers are used to couple light from laser diodes to an optical coupler and thence to a fiber to be tested, and from the coupler to a detector.
Such an OTDR, designed for testing single mode fibers and therefore itself including single mode fibers, is not suitable for testing of multi-mode fibers because this would involve a junction, at the connector for the fiber to be tested, between the single mode fiber of the OTDR and the multi-mode fiber to be tested. Although light would be propagated from the OTDR to the multi-mode fiber with relatively low loss, in the reverse direction the back-scattered light to be detected would suffer a large attenuation, typically greater than 10 dB, in passing from the multi-mode fiber to be tested to the single mode fiber of the OTDR. This large attenuation is due to the mismatch between the mode fields and numerical apertures of the two types of fiber, it being understood that the core diameter of the single mode fiber is typically 9 .mu.m, whereas the core diameter of the multi-mode fiber is in a range of about 50 to 100 .mu.m, typical core diameters for multi-mode fibers for communications applications being 50 .mu.m and 62.5 .mu.m. Accordingly, for testing multi-mode fibers it is necessary to provide an OTDR which itself uses multi-mode fibers for optical communications among its components.
Conversely, a multi-mode OTDR can not be used for testing single mode fibers, because again this would involve a junction at the connector of the OTDR between fibers of the two types, and consequently an excessive loss in propagating an optical signal from the multi-mode fiber of the OTDR into the single mode fiber to be tested.
It is therefore increasingly necessary for craftspersons to carry two distinct OTDRs, for testing single mode and multi-mode fibers respectively. This is inconvenient and expensive. In order to reduce these disadvantages, an OTDR may be provided with two separate plug-in optical units, for testing single mode and multi-mode fibers respectively, with a common signal processing and display unit. However, the disadvantages of inconvenience and expense remain.
An object of this invention, therefore, is to provide an improved OTDR which facilitates testing of either single mode and multi-mode fibers.